Reference Voltage Generator for Temperature Sensor with Trimming Capability at Two Temperatures

ABSTRACT

A temperature sensor circuit has a reference voltage generator that is trimmable at two temperatures for increased accuracy. The reference voltage generation section generates a reference voltage, the level of which is trimmable. A voltage divider section is connected to receive the reference voltage from the reference voltage generation section and generate a plurality of comparison voltage levels determined by the reference voltage and a trimmable resistance. An analog-to-digital converter can then be connected to a temperature dependent voltage section to receive the temperature dependent output voltage, such as a proportional to absolute temperature type (PTAT) behavior, and connected to the voltage divider section to receive the comparison voltage levels. The analog to digital converter generates an output indicative of the temperature based upon a comparison of the temperature dependent output voltage to the comparison voltage levels.

RELATED CROSS-REFERENCED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/175,226, filed Feb. 7, 2014, which is incorporated herein in itsentirety by this reference.

FIELD OF THE INVENTION

This invention pertains generally to the temperature sensor circuitsand, more particularly, to the trimming of such circuits.

BACKGROUND

Temperature sensor circuits can be used as peripheral circuits ondevices that use a temperature value or code to adjust their operations.For example, the bias levels on non-volatile memory circuits vary basedupon temperature. (For example, the temperature value from a temperaturesensor or digital thermometer, such as is described in U.S. Pat. No.7,889,575 or U.S. Pat. No. 8,228,739, is used for bias circuitry on anon-volatile memory circuit.) For accurate operation, the temperaturecode from such temperature sensors should be accurate. In practice, suchtemperature sensors have inaccuracy on their output due to tolerances inmanufacturing. Consequently, some kind of calibration is necessary inorder to correct the output (temperature code).

SUMMARY OF THE INVENTION

In a first set of aspects, a temperature sensor circuit includes atemperature dependent voltage section, a reference voltage generationsection, a voltage divider section, and an analog to digital converter.The temperature dependent voltage section provides an output voltagehaving a linear type dependence on temperature. The reference voltagegeneration section generates a reference voltage, where the level of thereference voltage is trimmable. The voltage divider section is connectedto receive the reference voltage from the reference voltage generationsection and generate from it a plurality of comparison voltage levelshaving a first level determined by the reference voltage and a secondlevel dependent upon a first trimmable resistance, the second levelbeing lower than the first. The analog to digital converter is connectedto the temperature dependent voltage section to receive the temperaturedependent output voltage and connected to the voltage divider section toreceive the comparison voltage levels. The analog to digital convertergenerates an output indicative of the temperature based upon acomparison of the temperature dependent output voltage to the comparisonvoltage levels.

Other aspects relate to a method of operating a temperature sensorcircuit. The temperature sensor circuit includes: a temperaturedependent voltage section providing a temperature dependent outputvoltage; a reference voltage generation section generating a referencevoltage; a voltage divider section connected to receive the referencevoltage from the reference voltage generation section and generatetherefrom a plurality of comparison voltage levels; and an analog todigital converter connected to receive the temperature dependent outputvoltage and the comparison voltage levels and generate therefrom anoutput indicative of the temperature. The method includes trimming at afirst temperature the level of the reference voltage based upon theoutput indicative of the temperature; and trimming at a secondtemperature a variable resistance in the voltage divider section basedupon the output indicative of the temperature, wherein the firsttemperature differs from the second temperature.

Various aspects, advantages, features and embodiments of the presentinvention are included in the following description of exemplaryexamples thereof, which description should be taken in conjunction withthe accompanying drawings. All patents, patent applications, articles,other publications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of terms between any of the incorporated publications,documents or things and the present application, those of the presentapplication shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a temperature sensor circuit.

FIG. 2 illustrates desired temperature sensor behavior.

FIG. 3 is an example of a trimmable reference voltage generationelement.

FIGS. 4 and 5 illustrate a trimming process for a circuit such as thatillustrated in the FIG. 3.

FIG. 6 is an exemplary embodiment of circuitry to generate the referencevoltages.

FIG. 7-10 illustrate an exemplary trimming process for the exemplaryembodiment.

DETAILED DESCRIPTION

The following considers temperature code error from temperature sensorsand the trimming of such sensors. An example of temperature sensorcircuit, such as is shown in FIG. 1, can be constructed using a band gapreference voltage generator section 101, which produces a voltage VPTATthat is proportional to absolute temperature (PTAT), a reference voltagegenerator section 103, which produces a voltage VREF, that is atemperature-independent reference voltage, and an analogue-to-digitalconverter (ADC) 105, that compares VPTAT voltage and VREF voltage toproduce a digital output (temperature code) at respective temperatures.The number of VREF voltages depends on the number of bits (5 in thisexample) of ADC, and each VREF voltage corresponds to each temperaturelevel, as shown in FIG. 2.

FIG. 2 illustrates the desired sort of behavior for the temperaturesensor circuit. In this example, the VPTAT rises linearly over the range−40 C to 110 C. The horizontal lines correspond to the reference valuesVREF<31:0> of the 5 bit, in this example, reference value, so that eachstep is something like 5 degrees C. Here the VREF value for 85 C, whichis a common reference value, is noted by the darker line.

As noted, FIG. 2 corresponds to the desired or ideal behavior. Inpractice, temperature sensors have inaccuracies on the digital outputdue to tolerances in manufacturing. Because of this, some kind ofcalibration is desirable in order to correct the digital output(temperature code). In one design, the VREF voltage generator has anability to adjust the VREF voltage so that digital output is correctedto the desired temperature code at a certain temperature. This can bedone by the Adjustment resistor 301 as shown in FIG. 3.

For instance, FIG. 4 illustrates the case that the VREF voltagecorresponding to 85 C does not match with VPTAT voltage at 85 C. For thesensor to be accurate, the VREF voltage corresponding to 85 C can beadjusted to match with VPTAT voltage at 85 C through the Adjustmentresistor 301 at the bottom of the voltage divider of FIG. 3. By trimmingthe voltage divider in this way, the VREF voltage corresponding to 85 Ccan match the VPTAT value at 85 C; however, this will also shift all ofthe VREF voltages corresponding to the other temperatures with thisadjustment, as shown in FIG. 5. As a consequence, away from the trimmingtemperature the VPTAT curve may be shifted out of the VREF range, asillustrated in FIG. 5 at −40 C. (Although the discussion here refers tomatching at 85 C, for example, as the temperature code steps in theexample are on the order of 5 C, in the case of a temperature codeoutput of 85 C, a more precise description is that the VPTAT voltage isin-between a VREF voltage which is equal to the VPTAT voltage at 82.5 Cand another VREF voltage which is equal to the VPTAT voltage at 87.5 C.)

To help overcome this limitation, an exemplary embodiment for a VREFgenerator can include two parts, as shown in FIG. 6 and has ability forVREF voltage adjustment at two different temperatures. A trimmablereference voltage generator section produces a VMAXTRIM voltage, whichis an input signal for the voltage divider and is a reference voltagetaken to correspond to the high trim level of, in this example, 85 degC. In this trimmable reference voltage generator, the output VMAXTRIM istaken from a node between the PMOS transistor 611 and a trimmableresistance connected in series between the supply level and ground. (Itshould be noted that VMAXTRIM is for the higher of the trim levels beingused, such as corresponding to 85 C in this example, which is not—or atleast not necessarily—corresponding to the highest of the temperaturesof the sensor.) Here, the trimmable resistance is formed of a trimmableportion 613 in series with a fixed portion 615. The gate of PMOS 611 iscontrolled by the output of an op-amp 601. One input of the op-amp is areference level, here BGR, and is taken to be largely temperatureindependent and the other input is taken from a node between thetrimmable resistance 613 and the fixed resistance 615 and reflects theoutput reference voltage VMAXTRIM. The variable resistance 613 can thenbe trimmed to set the VMAXTRIM voltage to correspond to the highertrimming level. The example of FIG. 6 illustrates a particularimplementation of an op-amp and output chain (PMOS 611, resistances 613and 615) is shown, but other implementation can be used.

To the right of FIG. 6, the voltage divider section supplies thereference voltages corresponding the VREF values for the temperaturevalues and uses the VMAXTRIM voltage as a reference voltage. As in FIG.1 or 3, the exemplary embodiment is again based on a 5 bit flash A to Dconverter and the reference voltages VREF<0> to VREF<31> are from nodesin a resistor chain formed between a PMOS 631 and a resistance formed ofa trimmable section Min Temp Trimming resistance 637 and a fixedresistance 639. The gate of PMOS 631 is now set by the output of anop-amp 621, which has one of its inputs connected to receive VMAXTRIMand has the other input connect to the node for VREF<xx> of the resistorchain that corresponds to the high trim value (85 C in this example). Aswith op-amp 601, the particular example shown for op-amp 621 is just onespecific implementation.

Under the arrangement of FIG. 6, the reference voltage generator hastrimming resistor, the Max Temp Trimming resistor 613, that adjustsVMAXTRIM voltage and also VREF voltage corresponding to the high trimlevel, or 85 deg C. in this case. The voltage divider section also hastrimming resistor, Min Temp Trimming resistor 637, that adjusts the VREFvoltage corresponding to lower trimming level, or −30 deg C. in thiscase. This allows for the both ends of the range to match. (As with theVMAXTRIM notation, it should be noted that “Min Temp Trimming” is forthe lower of the trim levels being used, not necessarily correspondingto the lowest of the temperatures of the sensor.)

An example of the trimming procedure can be illustrated with respect toFIGS. 7-10. For example, taking the case where the VREF valuecorresponding to 85 deg C. does not match with VPTAT voltage at 85 C, asshown in the FIG. 7, the VREF voltage corresponding to 85 C can beadjusted to match with VPTAT voltage at 85 C. The VREF voltage isadjusted with Max Temp Trimming resistor 613 in FIG. 6 so that the VREFvoltage corresponding to 85 C matches with VPTAT at 85 C in the VREFgenerator. The voltage difference between each VREF voltagecorresponding to each temperature is changed by adjusting VMAXTRIMvoltage, as shown in FIG. 8.

Although the first trimming process will set the voltage to the desiredhigh end of the temperature range, the VREF voltages corresponding tothe other temperatures may not be matched with VPTAT voltages at theother temperatures. After the first trimming at, in this example, 85 C,as shown in FIG. 9, it is desirable to do another trimming at anothertemperature. FIG. 9 (which is the same as FIG. 8, but with differentannotation) shows that VREF voltages corresponding to −30 C does notmatch with VPTAT voltage at −30 C, for instance.

In exemplary VREF generator, VREF voltage corresponding to, in thisexample, −30 C is adjusted with the Min Temp Trimming resistor 637, sothat it matches with VPTAT voltage at −30 C. As the VREF voltagecorresponding to 85 C is fixed by VMAXTRIM, it does not shift while theVREF voltage adjustment with Min Temp Trimming resistor 637 (FIG. 6), asshown in FIG. 10. As the VREF voltage adjustment is done at twodifference temperatures, other VREF voltages corresponding to the othertemperatures match with the VPTAT voltages at other temperatures.

The discussion above is based on a particular set of embodiments, butcan be applied more generally. For example, the analog to digitalconversion of the exemplary embodiments uses a flash type of A-to-Dconverter based on the values from the nodes of the resistor chain, butother implementation can readily be used. For example, a successiveapproximation, or SAR, A-to-D converter can be used.

The exemplary embodiments are also based on the temperature dependentvoltage level being linear in temperature (or at least a “linear-type”behavior, in that higher order behavior can be neglected and the voltageis sufficiently linear). In particular, the exemplary embodiment used aband gap reference type of circuit providing a proportional to absolutetemperature (PTAT) behavior, but other arrangements can be used. Forexample, an inverse proportional to absolute temperature behavior can beused, where the higher trimming level voltage would now correspond tothe lower trimming voltage and vice versa. In either case, circuits withtemperature dependent outputs other band gap based circuits can be used;and although a linear-type temperature dependence is most easilyimplemented for most other applications, other temperature dependencescan also be used.

For any of these embodiments or variations, the VREF generator hasability to adjust VREF voltages at two different temperatures. Whentrimming is done at just one temperature only, VREF voltagecorresponding to a particular temperature matches with VPTAT (or othertemperature dependent) voltage at that temperature; however, it may notbe matched at the other temperatures. With the ability to match at twotemperatures, the VREF voltages corresponding to the other temperaturescan also match with the PTAT voltage at the other temperatures.

Such reference voltage generating circuitry can be implemented asperipheral elements on integrated circuits for many applications. Forexample, the temperature value from a temperature sensor or digitalthermometer, such as is described in U.S. Pat. No. 7,889,575 or U.S.Pat. No. 8,228,739, is used for bias circuitry on a non-volatile memorycircuit, such as flash NAND memory and non-volatile memories having a 3Darray type structure. More detail on NAND memory devices can be found inUS patent and publication numbers 20080158969; U.S. Pat. Nos. 5,570,315;5,903,495; and 6,046,935, for example. More detail on non-volatilememory having a 3D array structure can be found in U.S. patentapplication Ser. Nos. 13/323,703; 14/153,794; and 14/149,601, forexample.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

It is claimed:
 1. A method, comprising: performing a trimming operationon a temperature sensor circuit in which a temperature dependent voltagesection provides a temperature dependent output voltage, a referencevoltage generation section generates a reference voltage, a voltagedivider section is connected to receive the reference voltage from thereference voltage generation section and generate therefrom a pluralityof comparison voltage levels, and an analog to digital converter isconnected to receive the temperature dependent output voltage and thecomparison voltage levels and generate therefrom an output indicative ofthe temperature, the trimming operation including: trimming at a firsttemperature the level of the reference voltage based upon the outputindicative of the temperature; and trimming at a second temperature avariable resistance in the voltage divider section based upon the outputindicative of the temperature, wherein the first temperature differsfrom the second temperature.
 2. The method of claim 1, wherein thetemperature dependent output voltage has a proportional to absolutetemperature behavior.
 3. The method of claim 1, wherein the firsttemperature is greater than the second temperature.
 4. The method ofclaim 1, wherein the second temperature is greater than the firsttemperature.
 5. The method of claim 1, wherein the trimming the level ofthe reference voltage includes setting a resistance value.
 6. The methodof claim 1, where the level of the reference voltage is trimmed prior totrimming the variable resistance in the voltage divider section.
 7. Themethod of claim 1, wherein the temperature dependent output voltage hasan inverse to proportional to absolute temperature type behavior.
 8. Themethod of claim 1, wherein the temperature dependent voltage section'soutput voltage is generated using a band gap circuit.
 9. The method ofclaim 1, wherein the voltage divider section includes a set of seriesconnected elements connected between a supply level and ground,including: a first transistor whose gate voltage is based on thereference voltage; a first trimmable resistance; and a plurality ofresistances connected in series, where the series of resistances isconnected to the supply level through the first transistor and connectedto ground through the first trimmable resistance, and wherein thecomparison voltage levels are taken from nodes between the elements ofthe set of series connected elements.
 10. The method of claim 9, whereinthe first transistor is a first PMOS transistor.
 11. The method of claim10, wherein the voltage divider section further includes: an op-amphaving a first input connected to receive the reference voltage, asecond input connected to a node of the set of series connectedelements, and an output connected to the gate of the first PMOStransistor.
 12. The method of claim 9, wherein the reference voltagegeneration section includes: an op-amp having a first input connected toreceive an additional reference level, a second input, and an output;and a PMOS transistor, a second trimmable resistance, and a fixedresistance connected in series between the supply level and ground,wherein the second input of the op-amp is connected to a node betweenthe second trimmable resistance and the fixed resistance, and the gateof the PMOS transistor is connected to the output of the op-amp.
 13. Themethod of claim 1, where the analog to digital converter is a flash-typeanalog to digital converter.
 14. The method of claim 1, where the analogto digital converter is a successive approximation type analog todigital converter.
 15. The method of claim 1, wherein the temperaturesensor circuit is formed as part of non-volatile memory circuit having aflash NAND type of structure for a memory array formed thereon.
 16. Themethod of claim 1, wherein the temperature sensor circuit is formed aspart of non-volatile memory circuit having a 3D type of structure for amemory array formed thereon.